Mr p bio 5 response to external stimuli

Question 1

how are multicellular organisms able to control the activities of different tissues and organs within their body?

Answer 1

by detecting stimuli and stimulating appropriate effectors

Question 2

how is responding to stimuli beneficial to animals?

Answer 2

it increases their chances of survival by avoiding harmful environments and by maintaining optimal conditions for their metabolism making them more likely to reproduce and raise offspring, so passing on the favourable allele

Question 3

what is a stimulus?

Answer 3

a detectable change in the internal or external environment of an organism that leads to a response

Question 4

what detects a stimulus?

Answer 4

receptors which are specific to one type of stimulus

Question 5

what causes a response to a stimulus?

Answer 5

effectors are cells that cause a response to a stimulus which include muscles and glands

Question 6

how do receptors communicate with effectors?

Answer 6

via the nervous and/or hormonal system

Question 7

what is the nervous system?

Answer 7

a rapid means of communication with a nervous system having many different receptors and effectors

Question 8

what is each receptor and effector linked to and why?

Answer 8

linked to a central coordinator of some type, which coordinates information between appropriate receptors and effectors

Question 9

what is a taxis?

Answer 9

a simple directional response where the direction of movement is determined by the direction of the stimulus. motile organisms respond by moving their whole body either towards or away.

Question 10

what are the names for moving towards or away from the stimulus during taxis?

Answer 10

towards (Positive taxis)
away (negative taxis)

Question 11

what is kinesis?

Answer 11

a non-directional response to a stimulus. once in a favourable condition, the organism slows down and turns more often

Question 12

why is kinesis useful?

Answer 12

because if an organism crosses a line between favourable to unfavourable conditions its chance of returning back to favourable conditions increase

Question 13

what is tropism?

Answer 13

the growth of part of a plant in response to a directional stimulus

Question 14

what are the types of tropism?

Answer 14

positive (towards) and negative (away)
phototropism (light), gravitropism (gravity)

Question 15

give 3 examples of stimuli that plants respond to

Answer 15

light, gravity, water

Question 16

what do plant responses to external stimuli involve?

Answer 16

“hormone-like substances” called plant growth factors

Question 17

what do the hormone like substances called plant growth factors do?

Answer 17

1. affect the growth and are produced by cells locatd throughout the plant rather than a specialised organ
2. plant growth factors effect the tissues that release them rather than acting on distant target organs

Question 18

what is an example of a plant growth factor?

Answer 18

indoleactic acid (IAA) which comes under the group of growth factors called auxins

Question 19

what does IAA control?

Answer 19

cell elongation among other things

Question 20

what are auxins?

Answer 20

a group of growth factors which speed up or slow down growth by controlling cell elongation

Question 21

where are auxins produced?

Answer 21

growing regions of the plant e.g. root and shoot tips

Question 22

what to auxins do in a plant?

Answer 22

have the ability to diffuse backwards to nearby tissue in order to cause cell elongation. in the shoots they stimulate growth but in the roots they inhibit growth

Question 23

what happens to auxin action if the shoot/root is removed?

Answer 23

no auxin will be available and the shoot stops growing

Question 24

where is IAA made and where does it move to?

Answer 24

made in the tip of the root or shoot. As it diffuses into the growing region, it binds to protein receptors on cell membranes

Question 25

how does IAA work?

Answer 25

causes H+ to be released which lowers pH and affects bonding, including the H bonds between microfibrils of cellulose. As a result, the plant cell walls weaken and the plant cells can be easily stretched and elongate

Question 26

what is the process of phototropism in flowering plants? (shoots)

Answer 26

1. cell in shoot tip produce IAA which diffuses backwards
   2.IAA initially is transported evenly throughout all regions
2. light causes IAA to move from the light side to the shaded side
3. greater conc of IAA on the shaded side vs the light side
   5.IAA causes elongation of shoot cells, so shaded side cells elongate more
   6.shaded side elongated faster than the light side causing the shoot to bend towards the light

Question 27

describe phototropism in flowering plants (Roots)

Answer 27

-in roots, high conc of IAA inhibits cell elongation if a root comes into contact with light (from above):

1. the IAA moves to the sahded side
2. inhibits cell elongation
   3.topside elongated faster
   4.root grows away from the light source

Question 28

how do plants cells detect the direction of gravity?

Answer 28

columella cells near the root tip contain organelles called amyloplasts which are packed with starch
they are heavy and sink to the bottom of cells they are in. this enables cells to detect the direction of gravity
IAA is actively transported to the side of the root to which the amyloplasts sink

Question 29

describe gravitropism (root)

Answer 29

1. cells in the tip produce IAA which diffuses backwards
2. IAA is initially evenly spread
3. Gravity influences the movement of IAA from the upper side to the lower side
4. greater conc of IAA on lower side
5. IAA inhibits elongation in roots- lower side elongates less than the upper side
   6.root bends downwards towards the force of gravity

Question 30

describe gravitropism int the shoots

Answer 30

in shoots, greater conc of IAA on the lower side, increase cell elongation, causing the lower side to elongate faster- moves away from the force of gravity